Authors of a new study published in last month’s edition of the Journal of Health Affairs found that White women lacking a high school diploma experienced a five-year decline in average life expectancy from 1990-2008, the largest of any demographic in the report “Differences In Life Expectancy Due to Race and Educational Differences Are Widening, And Many May Not Catch Up.”

When it comes to health in the U.S., researchers say there is a sharp divide in average life expectancy based upon income. Those with higher incomes and education tend to live longer than those who don’t. It’s a statistical reality that has been observed by medical researchers for many years.

The exact reasons for this sharp decline, however, have baffled researchers, including S. Jay Olshansky, the study’s lead researcher. “We actually don’t know the exact reasons why it’s happened,” said Olshansky, who studies human longevity at the University of Illinois-Chicago. “I wish we did.”

Olshansky notes that such a drop is almost unheard of for any demographic in the history of the U.S. Although racial minorities and poorer Americans have historically lagged behind White middle class citizens, gains or declines for any group are usually small and gradual.

“If you look at the history of longevity in the United States, there have been no dramatic negative or positive shocks,” Olshansky says. “With the exception of the 1918 influenza pandemic, everything has been relatively steady, slow changes. This is a five-year drop in an 18-year time period. That’s dramatic.”

The usual culprits, including poor diet, heart disease, stress and limited medical care are the likely causes of this growing, sudden gap. The American Prospect reports that the principle cause could be economy itself. With fewer opportunities for those without high school diplomas, declining health is likely a result of poverty itself.

“Hope is lowered. If you drop out of school, say, in the last 20 years or so, you just had less hope for ever making it and being anything,” said James Jackson, a public-health researcher at the University of Michigan. “The opportunities available to you are very different than what they were 20 or 30 years ago. What kind of job are you going to get if you drop out at 16? No job.”

Declining health for low-income women is not limited to the U.S. and is part of a troubling trend for health researchers who have observed similar poor health for women in developing countries.

A recent study by the World Health Organization found that as the average life expectancy grows for many developing countries, the gap in life expectancy for women in rich and poor countries is growing apart, much like within the U.S.

“The fact that non-communicable diseases strike these women at an earlier age in less developed countries has major implications, as these deaths are devastating for individuals, families and societies,” said study co-author Dr. John Beard, director of the WHO’s Department of Ageing and Life Course.